Meson build instructions for Tracker

See <http://mesonbuild.com/> for information about Meson.
Remaining issues:
* There's no `make dist` equivalent. We currently produce release
tarballs containing the output files of the Vala compiler. We need
to think through whether we can stop doing that. Shipping the
generated .c files does make the Vala preprocessor useless so
it would be good if we can stop.
* The Firefox, Thunderbird, Evolution and Nautilus plugins are not
built
* https://github.com/mesonbuild/meson/issues/671 -- means we can't
depend on tracker_common_dep in most places and have to manually
set link_with, include_directories and sources instead.
* https://github.com/mesonbuild/meson/issues/1469 -- without this we
have to install generated Vala headers using a script
* https://github.com/mesonbuild/meson/issues/1229 -- means adding
the #include guard to libtracker-sparql/tracker-generated-no-checks.h
is a pain
* The test suite has some spurious failures.
Here's a rough speed comparison.
Meson:
time sh -c 'meson .. --prefix=/opt/tracker-meson -D bash_completion=/opt/tracker-meson/share/bash-completion -Dsystemd_user_services=/opt/tracker-meson/lib/systemd/user -Ddbus_services=/opt/tracker-meson/share/dbus-1/services && ninja-build -j 4 && ninja-build install'
real 1m8.194s
user 2m16.962s
sys 0m20.532s
Autotools:
time sh -c './configure --prefix=/opt/tracker-autotools --with-bash-completion-dir=/opt/tracker-autotools/share/bash-completion --with-session-bus-services-dir=/opt/tracker-autotools/share/dbus-1/services --disable-nautilus-extension && make -j 4 && make install'
real 2m37.750s
user 4m37.214s
sys 0m54.806s
Plus 30+ seconds of ./autogen.sh first.
Note that Meson builds may fail if your source tree has generated files
from an Autotools build in there. If you see errors about duplicate
definitions, first try cleaning your source tree (use `git clean -dfx`,
but make sure you commit any work first!!)